632 - 634 Abu Bakr, one of the Prophet's first converts and his father-in-law (one of Muhammad's wives, Aisha, was his daughter), established the caliphate (khilaafa, or, "succession"), initiating the first dynasty of caliphs (sing. khalifa, plural, khulafaa ).The Caliph would become known as "God's shadow on earth." The first four caliphs became known as al-rashidoon ("the rightly-guided ones"), and they ruled from their capital in Medina. All four were, like the Prophet himself, members of the leading clan of Mecca, the Quraysh, and, thus, were close relatives of the Prophet. The period of the Muslim conquest dates from this time. Abu Bakr conducted the Ridda ("Apostasy") Wars against those Arabian tribes who had questioned his authority. He also sent Muslim armies into Syria and Iraq. Scholars debate about whether it is more proper to refer to the expansion of Muslim power as a jihad ("striving," and "holy war"), or as a hijra, ("migration"). In any case, resistance was minimal: the Byzantine Empire was too weak and torn with internal strife (see Councils of Nicea and Chalcedon). The caliphate would last until 1924 when Ataturk abolished it. It would become the goal of Osama bin Laden and other radical jihadists in the twenty-first century to restore it.

One of Abu Bakr's first acts as Caliph was to disinherit Ali (Muhammad's cousin) and Ali's wife Fatima (Muhammad's daughter by his first wife Khadija)
from the Prophet's estate. From that point on, the Prophet's family (ahl al-bayt - "People of the House") was to be supported only through alms. Abu Bakr, a member of the
Qurayshi Banu ("sons of") Taim clan, used his daughter Aisha's status as the Prophet's widow as a lever to shift power away from the clan of Muhammad and Ali, the Banu Hashim, a clan holding relatively minor status within the Quraysh tribe, back toward the more prestigious clans.
Aisha (ca. 613-678) had been betrothed to Muhammad when she was only seven and was only eighteen when he died.
Aisha’s antipathy for Ali seems to have begun when Ali advised his cousin the Prophet to divorce her (there were suspicions she had been adulterous). Out of this power struggle between clans related to Muhammad grew Islam's fitna ("civil strife") and the split into sunni ("orthodox" - Abu Bakr's side) and shia ("partisans" for Ali). (more on shia) (see also "Battle of the Camel.") (see Reza Aslan, No god but God: The Origins, Evolution, and Future of Islam (New York: Random House, 2005), 118 ff.)

634 - 644 Caliphate of Umar Ibn al-Khattab, handpicked successor to Abu Bakr. Most of the Islamic "conquest" took place during his time. In 644, he was murdered by a Christian slave.

634 Umar's army defeats an army of the Byzantines in southern Syria.

635 Muslim army captures Damascus.

636 Battle of Yarmuk: The Byzantine army, far larger in number, was defeated by a much smaller Muslim force. Muslim sovereignty over Palestine begins. This victory would provide fodder for Muslim jihadist propaganda down into contemporary times.

637 Battle of Qadisiyya: The Sasanian army was defeated in Mesopotamia by a Muslim army much inferior in number. After the Muslim victories over the Byzantine and Sasanian armies, Palestine, Syria, Iraq, Egypt, and Persia fell under Muslim control. In 1980, Iraq's ruler Saddam Hussein would call the war he started with Iran (First Gulf War) the "second Qadisiyya."

638 The "Covenant of Umar," a pact between Umar Ibn Khatib and the Christians of Jerusalem, was concluded on the occasion of the conquest of that city by the Muslims. Umar decreed that Muslims should forever thereafter guarantee Christians freedom of religion, use of their churches for worship, and the right to visit holy places. In another version, Umar rescinded the Roman decrees that had banished Jews from Jerusalem and accorded Jews all the rights granted Christians.

Non-Muslims were not required to participate in jihad (military action in defense of Islam) nor did they have to pay the zakat (the tax for charity required of all Muslims), but they were required to pay the jizya, a poll tax that helped defray the expense of protecting them. Since Muslim taxes were significantly lower than those imposed by the Byzantines and since Muslims allowed citizens more latitude to pursue their own customs and religious beliefs, Jews and Christians almost universally welcomed their new rulers.

Also in 639, Arab invaders led by Abu Musa al-Ashari conquered Khuzestan (southwest province in modern Iran -- see also).

643 The Muslim general Amr Ibn al-As conquered Libya in the name of Islam.

644 - 656 Caliphate of Uthman Ibn Affan. The Qur'an, as we know it, was edited in his time but would not reach final form until as late as 935. By the time Uthman assumed the caliphate, confusion had begun to abound over which of at least four different existing texts was the definitive word of God as transmitted to Muhammad. Muslims traditionally have believed that the Prophet himself was illiterate and that he had dictated the Qur'an to his followers. They memorized it, the normal method of transmission for Arabic poetry. Only after the Prophet's death did it occur to anybody to write it down. The rapidly escalating confusion prompted Uthman to appoint a committee in 650 to come up with a single, authoritative text. The Prophet's secretary, Zaid Ibn Thabit, served on that committee.

But, the new edition of the Qur'an became the Caliph's undoing. Uthman met his end in 656, murdered while praying by rebellious Egyptians who, along with rebels from Iraq and Syria, were incensed because they felt Uthman had overstepped his authority when he ordered that their variant editions of the Qur'an be destroyed. Mu'awiyya, Umayyad governor of Syria, hearing of the threats against his uncle the Caliph had sent soldiers to rescue him, but they arrived too late. This event provided the pretext for the first battle in thefitna ("dissension"), or, civil war, in Islam. One of the earliest dissident groups in Islam, the Kharijites, emerged at this time.

This was a turning point. After this time, the unified, brotherly body of believers (the umma) that Muhammad had sought to create and which apparently did mark the earliest years, lived on only in the theology, nostalgic memory, and hopes of Muslims, but, never again in their history.

656 - 661 Caliphate of Ali Ibn Talib, cousin of the Prophet, also son-in-law by virtue of his marriage to Muhammad's daughter, Fatima. Ali, however, refused the title "Caliph," choosing instead the title, Amir al-Mu'manin ("Commander of the Faithful"). Ali moved the Arab capital from Medina to Kufa in southern Iraq. His sons were Husayn (martyred in 680), and Hasan (who died in 669). During Ali's caliphate, the split in the Muslim umma ("community") intensified over the question, should a direct familial descendant of Muhammad, or, a selected leader from his tribe become caliph? The two chief sects in Islam appeared at this point: the Shiites ("partisans," i.e. partial to the caliph Ali and his descendants through Husayn) favoring the first option (today only 15% of all Muslims), and the Sunni ("orthodox") favoring the second path. (see also origins of sunni-shia fitna).

Ali angered the Kharijites when, following an inconclusive military engagement with the Umayyads, the Battle of Siffin in 658, he agreed to submit his dispute with the Umayyads to arbitration, a process that was decided in favor of the more powerful party, the Umayyads. Ali was murdered by the Kharijites in Kufa in 661 and buried in the nearby city of Najaf, a major shrine for Shiites.

656 Islam's first military episode in its fitna ("civil war") came with the "Battle of the Camel," fought in Basra (southern Iraq) and so named because Aisha, widow of Muhammad, led her troops into battle against Ali riding one. Aisha (daughter of Abu Bakr, the first Caliph) did not support Ali in his claim to become Caliph. Aisha and her allies were defeated by Ali's forces. This event marked the emergence of the two most populous sects in Islam: Sunni ("orthodox"), who by our own day claimed 85% of all Muslims worldwide over against Shia or Shiite ("partisans") Muslims, who made up most of the remaining 15%.

658 Ali appointed his loyal companion Malik al-Ashtar governor of Egypt. He wrote a letter to Malik on how to govern wisely. This letter became one of the models of Muslim jurisprudence and was preserved as "Letter 53" in a collection of the sermons and letters of Ali known as Nahj al-Balagha ("The Peak of Eloquence"). Here is an excerpt from Ali's letter to Malik:

"Do not behave toward them [the people] like a beast of prey and do not seize that which belongs to them. Remember
that they are of two kinds: they are either your brothers in religion or your brothers in creation...Let your mercy and compassion come to their rescue and help in the same way and to the same extent that you expect Allah to show mercy and forgiveness to you."